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Hardness

The resistance of a substance to surface abrasion.

Head

An end closure for the filter case or bowl, which contains one or more ports.

Heat exchanger

A device, which transfers heat through a conducting wall from one fluid to another.

Housing

A ported enclosure, which directs the flow of fluid through the filter element.

Hydraulic Fluid

Fluid serving as the power transmission medium in a hydraulic system. The most commonly used fluids are petroleum oils, synthetic lubricants, oil-water emulsions, and water-glycol mixtures. The principal requirements of a premium hydraulic fluid are proper viscosity, high viscosity index, anti-wear protection (if needed), good oxidation stability, adequate pour point, good demulsibility, rust inhibition, resistance to foaming, and compatibility with seal materials. Anti-wear oils are frequently used in compact, high-pressure, and capacity pumps that require extra lubrication protection.

Hydraulic Oil

An oil specially suited for use as either the specific gravity or the API gravity of a liquid.

Hydraulics

Engineering science pertaining to liquid pressure and flow.

Hydrocarbons

Compounds containing only carbon and hydrogen. Petroleum consists chiefly of hydrocarbons.

Hydrodynamic lubrication

A system of lubrication in which the shape and relative motion of the sliding surfaces causes the formation of a fluid film having sufficient pressure to separate the surfaces.

Hydro finishing

A process for treating raw extracted base stocks with hydrogen to saturate them for improved stability.

Hydrolysis

Breakdown process that occurs in anhydrous hydraulic fluids as a result of heat, water, and metal catalysts (iron, steel, copper, etc.)

Hydrolytic stability

Ability of additives and certain synthetic lubricants to resist chemical decomposition (hydrolysis) in the presence of water.

Hydrometer

An instrument for determining either the specific gravity of a liquid or the API gravity.

Hydrostatic lubrication

A system of lubrication in which the lubricant is supplied under sufficient external pressure to separate the opposing surfaces by a fluid film.

Hypoid gear lubricant

A gear lubricant having extreme pressure characteristics for use with a hypoid type of gear as in the differential of an automobile.